Geography Reference
In-Depth Information
High pressure is so-named because the atmosphere is pressing down on the Earth. In contrast, low
pressure is so-named because, due to its upward-moving air, the pressure (or weight) of the atmo-
sphere against the Earth is comparatively low. Both are linked in a three-dimensional pattern of at-
mospheric circulation as shown in Figure 9-9.
Figure 9-9:
A
cross-section of
high- and low-at-
mospheric
pressure-sys-
tems.
Solar energy sets this circulatory system in motion. Some parts of Earth heat up more rapidly than
others. Over those areas that do, the air tends to warm, expand, and rise. The vapor in the air cools
as it rises in a
convection current,
causing condensation and (in all probability) precipitation to occur.
Thus, low pressure is associated with cloudy, rainy (or snowy) conditions.
After precipitating, air at the top of a low-pressure system is cool, dry (having “lost” its moisture) and
heavy. It wants to sink back down to Earth, but can't because of other air coming up from underneath.
Air in the upper atmosphere therefore moves laterally until it finds a place where it can descend as a
high-pressure system composed of comparatively cool, clear, and dry (low humidity) air.
Pressure belts
Because the equatorial latitudes receive a greater degree of solar energy than anyplace else on Earth,
a global “belt” of low-atmospheric pressure characterizes them (see Figure 9-10). This phenomenon
is called the
inter-tropical convergence zone
(ITCZ), because air from the tropics north and south of
the equator is drawn into (converges on) this zone before it rises in a convection current.
The result is a warm, humid “rainmaker” that produces the tropical climates presented in Chapter 10.
As implied by Figure 9-9, the air that rises in this low-pressure belt must fall to Earth elsewhere. Gen-
erally, this occurs in two sub-tropical high-pressure belts that roughly correspond to Latitudes 25-30
North and South. Given these belts of “drought-makers,” it's not surprising to see desert and semi-
desert conditions over much of these latitudes.
